1. Field of the Invention
The present invention relates to piezoelectric or pyroelectric polymer materials. It provides a manufacturing process comprising the cross-linking of the chains forming the polymer, which results in improving some of its piezoelectric characteristics and conferring thereon a better dimensional stability with respect to temperature variations.
2. Description of the Prior Art
Some polymers are capable of acquiring piezoelectric or pyroelectric properties when they are subjected to mechanical or electrical stresses. This is the case more especially with vinylidene polyfluoride (PVF.sub.2) which is one of the most widely used. Piezoelectric properties are conferred on this material by two successive or simultaneous treatments. These treatments consist in stretching a sheet of PVF.sub.2 by a factor equal at least to three, at a temperature of about 70.degree. C., and subjecting it to an intense electric field greater than or equal to 1 MV/cm. These operations may be simultaneous as in the case of laminating with polarization. The induced deformation is as a rule irreversible, except round about the melting temperature, for it is accompanied by a spherulitic fibrillar morphological transition. However, it has been discovered that a part of this deformation is recovered by heating beyond the laminating temperature: for example, re-heating to 90.degree. C. for one hour without applied stresses leads to a shrinkage of 7 to 8% in the stretching direction and substantially the same relative increase in thickness: volume and width (perpendicular to the stretching direction) remain constant. This re-heating gives to the polymer sheet, at temperatures which may be higher than that corresponding to the operations for acquiring its piezo- or pyroelectric properties, dimensional stability in time and a stable behavior of the piezo- or pyroelectric activity. The stability of these parameters is achieved however to the detriment of the piezoelectric activity which drops from 10 to 30%. Other disadvantages also appear. For example, the frequencies corresponding to natural vibration modes are affected, whether the sample is free (variations of dimensions) or partially clamped (variations of the internal mechanical tension state).